Systems of pipes and tubes (hereinafter "tubes") are frequently utilized in oil and gas refineries and in chemical processing plants. In many applications, tubing is used not only as a conduit for transporting fluids to be processed, but also as an integral component of processing equipment. Additionally, such furnace tubes may also contain catalyst to cause a catalytic reaction to occur as the fluid flows therethrough. Such furnace tubes typically experience substantial thermal expansion, which may create bending stresses on the tubes and, particularly, at joints where the furnace tubes are connected to other tubes, such as "cross-over" tubes which may be used to carry heated fluid from the furnace tubes to headers. Because such cross-over tubes are not exposed to as much heat as the furnace tubes are exposed to, and because cross-over tubes must be somewhat flexible to accommodate the thermal expansion of the furnace tubes, cross-over tubes are typically provided with thinner walls than furnace tubes. As a result, cross-over tubes typically experience greater bending stress than furnace tubes. Consequently, the joint between cross-over tubes and furnace tubes is particularly susceptible to failure and typically is a weak point in the system of tubes. A failure can result in the leakage of heated hydrocarbons which can result in fires and explosions and, as a result, can be very dangerous and require an unplanned and costly shut down of the furnace and units associated with the furnace so that the failure may be repaired.
Typically, joints between furnace tubes and cross-over tubes are welded together. Furthermore, they are commonly reinforced by welding a fitting, such as a sockolet, onto the side of the furnace tube so that the cross-over tube may be fitted into a recess in the sockolet and welded thereto. When the cross-over tube is welded to the sockolet, however, heat from the weld operation creates residual stress in the portion of the tube proximate to the weld, reduces the allowable stress of that portion of the tube, and increases the stress concentration at that portion of the tube. To compensate for these consequences, a stress collar may be fitted about the cross-over tube to reinforce it. To enable the stress collar to readily fit over the cross-over tube, the stress collar is generally sized to provide a small gap of, for example, approximately 10-20 mills (i.e., 0.01-0.02 inches) between the inside diameter of the collar and the outside diameter of the cross-over tube. The purpose of the stress collar, however, is largely defeated because, as a result of the gap, when the header thermally expands and causes the cross-over tube to deflect, the deflection is not constrained by the stress collar and a bending moment and a resulting bending stress induced in the cross-over tube is not distributed and absorbed by the stress collar as intended. As a consequence, the reduction of the failure rate of cross-over tubes using stress collars is minimal at best and, often, the failure rate is actually increased, and the dangers and costs discussed above are increased.
In an alternative attempt to reduce joint stresses and failures, expansion joints may be used to join furnace tubes to cross-over tubes. Expansion joints, however, are very costly and, while they reduce the stresses that the cross-over tubes are exposed to, they are still susceptible to leakage and the resultant dangers and costs associated therewith as discussed above.
As a result of the potential danger and cost of joint failures, a continuing search has been directed to the development of tube joints that can withstand relatively high thermal stresses.